Understanding wind pressure coefficients: why windward and leeward walls see different pressure
Wind doesn't press on a building with one uniform pressure — the wall it hits head-on and the wall on the far side experience genuinely different physical effects, captured by two different pressure coefficients.
The wall a wind stream hits directly — the windward wall — is where the moving air is brought essentially to a stop against the building's surface, converting its kinetic energy directly into a positive pressure pushing inward. Both EC1 and ASCE 7 treat this as a near-constant coefficient (cpe = +0.8, Cp = +0.8) regardless of the building's proportions, because that direct stagnation effect doesn't vary much with plan shape.
The leeward wall: suction, and why its magnitude depends on building proportions
The wall on the far side, the leeward wall, experiences something physically different: as wind flows around and over the building, it separates from the surface downstream, creating a low-pressure wake that pulls outward on that wall — suction, a negative pressure — rather than pushing in. Unlike the windward coefficient, this leeward coefficient genuinely depends on the building's proportions in the wind direction — EC1 tabulates it against h/d (height over depth in the wind direction), ASCE 7 against L/B (length over breadth) — because a more slender building in that direction disturbs the wind flow differently than a squat one, changing how strong that wake suction is.
What actually loads the structure combines this outward-facing external pressure (positive on the windward face, suction on the leeward face) with an internal pressure that also acts on every interior surface, from openings, leakage, or a deliberately partially-enclosed design — so windward and leeward faces end up adding together rather than partly cancelling: the windward wall gets pushed in, the leeward wall gets pulled out, both contributing to the same overall along-wind force on the structure. Wind Load Calculator computes exactly this pair of pressures directly, accounting for how wind speed itself increases with height above the ground, and combines them into a net along-wind force for whichever internal-pressure case governs — covering the windward and leeward walls of the building's Main Wind-Force Resisting System specifically, not side walls, roof pressures, or the separate local pressures used for cladding and fastener design (a genuinely different design case, covered next) (see the related tool below).